System x(c)- and thioredoxin reductase 1 cooperatively rescue glutathione deficiency

Cystine transporter SLC7A11/xCT in cancer: ferroptosis, nutrient dependency, and cancer therapy

The cystine/glutamate antiporter SLC7A11 (also commonly known as xCT) functions to import cystine for glutathione biosynthesis and antioxidant defense and is overexpressed in multiple human cancers. Recent studies revealed that SLC7A11 overexpression promotes tumor growth partly through suppressing ferroptosis, a form of regulated cell death induced by excessive lipid peroxidation. However, cancer cells with high expression of SLC7A11 (SLC7A11^high) also have to endure the significant cost associated with SLC7A11-mediated metabolic reprogramming, leading to glucose- and glutamine-dependency in SLC7A11^high cancer cells, which presents potential metabolic vulnerabilities for therapeutic targeting in SLC7A11^high cancer. In this review, we summarize diverse regulatory mechanisms of SLC7A11 in cancer, discuss ferroptosis-dependent and -independent functions of SLC7A11 in promoting tumor development, explore the mechanistic basis of SLC7A11-induced nutrient dependency in cancer cells, and conceptualize therapeutic strategies to target SLC7A11 in cancer treatment. This review will provide the foundation for further understanding SLC7A11 in ferroptosis, nutrient dependency, and tumor biology and for developing novel effective cancer therapies.

Emerging mechanisms and applications of ferroptosis in the treatment of resistant cancers

The development of chemotherapy drugs has promoted anticancer treatment, but the effect on tumours is not clear because of treatment resistance; thus, it is necessary to further understand the mechanism of cell death to explore new therapeutic targets. As a new type of programmed cell death, ferroptosis is increasingly being targeted in the treatment of many cancers with clinical drugs and experimental compounds. Ferroptosis is stimulated in tumours with inherently high levels of ferrous ions by a reaction with abundant polyunsaturated fatty acids and the inhibition of antioxidant enzymes, which can overcome treatment resistance in cancers mainly through GPX4. In this review, we focus on the intrinsic cellular regulators against ferroptosis in cancer resistance, such as GPX4, NRF2 and the thioredoxin system. We summarize the application of novel compounds and drugs to circumvent treatment resistance. We also introduce the application of nanoparticles for the treatment of resistant cancers. In conclusion, targeting ferroptosis represents a considerable strategy for resistant cancer treatment.

Auranofin mitigates systemic iron overload and induces ferroptosis via distinct mechanisms

Iron homeostasis is essential for health; moreover, hepcidin-deficiency results in iron overload in both hereditary hemochromatosis and iron-loading anemia. Here, we identified iron modulators by functionally screening hepcidin agonists using a library of 640 FDA-approved drugs in human hepatic Huh7 cells. We validated the results in C57BL/6J mice and a mouse model of hemochromatosis (Hfe^−/− mice). Our screen revealed that the anti-rheumatoid arthritis drug auranofin (AUR) potently upregulates hepcidin expression. Interestingly, we found that canonical signaling pathways that regulate iron, including the Bmp/Smad and IL-6/Jak2/Stat3 pathways, play indispensable roles in mediating AUR’s effects. In addition, AUR induces IL-6 via the NF-κB pathway. In C57BL/6J mice, acute treatment with 5 mg/kg AUR activated hepatic IL-6/hepcidin signaling and decreased serum iron and transferrin saturation. Whereas chronically treating male Hfe^−/− mice with 5 mg/kg AUR activated hepatic IL-6/hepcidin signaling, decreasing systemic iron overload, but less effective in females. Further analyses revealed that estrogen reduced the ability of AUR to induce IL-6/hepcidin signaling in Huh7 cells, providing a mechanistic explanation for ineffectiveness of AUR in female Hfe^−/− mice. Notably, high-dose AUR (25 mg/kg) induces ferroptosis and causes lipid peroxidation through inhibition of thioredoxin reductase (TXNRD) activity. We demonstrate the ferroptosis inhibitor ferrostatin significantly protects liver toxicity induced by high-dose AUR without comprising its beneficial effect on iron metabolism. In conclusion, our findings provide compelling evidence that TXNRD is a key regulator of ferroptosis, and AUR is a novel activator of hepcidin and ferroptosis via distinct mechanisms, suggesting a promising approach for treating hemochromatosis and hepcidin-deficiency related disorders.

Redox Modulation and Induction of Ferroptosis as a New Therapeutic Strategy in Hepatocellular Carcinoma

Ferroptosis, a newly discovered form of cell death mediated by reactive oxygen species (ROS) and lipid peroxidation, has recently been shown to have an impact on various cancer types; however, so far there are only few studies about its role in hepatocellular carcinoma (HCC). The delicate equilibrium of ROS in cancer cells has found to be crucial for cell survival, thus increased levels may trigger ferroptosis in HCC. In our study, we investigated the effect of different ROS modulators and ferroptosis inducers on a human HCC cell line and a human hepatoblastoma cell line. We identified a novel synergistic cell death induction by the combination of Auranofin and buthionine sulfoxime (BSO) or by Erastin and BSO at subtoxic concentrations. We found a caspase-independent, redox-regulated cell death, which could be rescued by different inhibitors of ferroptosis. Both cotreatments stimulated lipid peroxidation. All these findings indicated ferroptotic cell death. Both cotreatments affected the canonical ferroptosis pathway through GPX4 downregulation. We also found an accumulation of Nrf2 and HO-1, indicating an additional effect on the non-canonical pathway. Our results implicate that targeting these two main ferroptotic pathways simultaneously can overcome chemotherapy resistance in HCC.

Usefulness of bile as a biomarker via ferroptosis and cysteine prenylation in cholangiocarcinoma; role of diagnosis and differentiation from benign biliary disease

BACKGROUND

Cholangiocarcinoma (CCA) is a malignant cancer of the biliary tract with a poor prognosis. Herein, we investigated possible mechanism of extrahepatic CCA (eCCA) by dysregulated iron metabolism and post-translational modifications (PTMs). Further, we evaluated potential biomarkers in the bile fluid for diagnosis of eCCA and differentiation between eCCA and benign biliary disease.

METHODS

From August 2018 to April 2019, we obtained bile fluids from 46 patients; 28 patients with eCCA (eCCA group) and 18 patients with common bile duct stone (Control group) via percutaneous transhepatic biliary drainage. We examined the levels of reduced glutathione (GSH), peroxide, ferrous iron [Fe⁺²], glutathione peroxidase (GP_X) and farnesyl transferase/geranylgeranyl transferase type-1 subunit alpha (FNTA) concentration in bile fluids to clarify the mechanism of ferroptosis and prenylation.

RESULTS

The remarkable difference of PTMs was that FNTA which means prenylated cysteine as regulator was significantly decreased in eCCA than that of control. In addition, level of GSH, peroxide, GP_X and ferrous iron [Fe⁺²] were significantly depleted in eCCA than control. These results demonstrate that PTM, dysregulated iron metabolism and GP_X-regulated ferroptosis with GSH depletion through cysteine modification in bile are possible mechanisms of eCCA. Liquid Chromatography (LC)-Mass Spectrometry (MS) analysis, several oncogenic pathways including MYC target, apoptosis, fatty acid metabolism, P53 and mTORC1 were enriched in eCCA.

CONCLUSIONS

In conclusion, redox-dependent modification of cysteine and ferroptosis in bile fluids are possible mechanisms of eCCA. Several protein and oncogenic pathways related to PTM which are seen in eCCA tissues were also enriched in bile fluids. It suggests that bile fluid represents the oncogenic characteristics of eCCA tissues. Therefore, bile fluids have a role of a biomarker for diagnosis in eCCA, especially, differentiation of eCCA from benign biliary stricture.

The Complex Interplay between Antioxidants and ROS in Cancer

Reactive oxygen species (ROS) play important roles in tissue homeostasis, cellular signaling, differentiation, and survival. In this review, we discuss the types ofROS, their impact on cellular processes, and their pro- and antitumorigenic effects. Further, we discuss recent advances in our understanding of both endogenous and exogenous antioxidants in tumorigenic processes. Finally, wediscuss how aberrant activation of antioxidant programs by the transcription factor NFE2-related factor 2 (NRF2) influences tumorigenesis and metastasis, and where the current gaps in our knowledge remain.

Achieving Life through Death: Redox Biology of Lipid Peroxidation in Ferroptosis

Redox balance is essential for normal brain, hence dis-coordinated oxidative reactions leading to neuronal death, including programs of regulated death, are commonly viewed as an inevitable pathogenic penalty for acute neuro-injury and neurodegenerative diseases. Ferroptosis is one of these programs triggered by dyshomeostasis of three metabolic pillars: iron, thiols, and polyunsaturated phospholipids. This review focuses on: (1) lipid peroxidation (LPO) as the major instrument of cell demise, (2) iron as its catalytic mechanism, and (3) thiols as regulators of pro-ferroptotic signals, hydroperoxy lipids. Given the central role of LPO, we discuss the engagement of selective and specific enzymatic pathways versus random free radical chemical reactions in the context of the phospholipid substrates, their biosynthesis, intracellular location, and related oxygenating machinery as participants in ferroptotic cascades. These concepts are discussed in the light of emerging neuro-therapeutic approaches controlling intracellular production of pro-ferroptotic phospholipid signals and their non-cell-autonomous spreading, leading to ferroptosis-associated necroinflammation.

Irreversible TrxR1 inhibitors block STAT3 activity and induce cancer cell death

Because of its key role in cancer development and progression, STAT3 has become an attractive target for developing new cancer therapeutics. While several STAT3 inhibitors have progressed to advanced stages of development, their underlying biology and mechanisms of action are often more complex than would be expected from specific binding to STAT3. Here, we have identified and optimized a series of compounds that block STAT3-dependent luciferase expression with nanomolar potency. Unexpectedly, our lead compounds did not bind to cellular STAT3 but to another prominent anticancer drug target, TrxR1. We further identified that TrxR1 inhibition induced Prx2 and STAT3 oxidation, which subsequently blocked STAT3-dependent transcription. Moreover, previously identified inhibitors of STAT3 were also found to inhibit TrxR1, and likewise, established TrxR1 inhibitors block STAT3-dependent transcriptional activity. These results provide new insights into the complexities of STAT3 redox regulation while highlighting a novel mechanism to block aberrant STAT3 signaling in cancer cells.

Amino acids in cancer

Over 90 years ago, Otto Warburg's seminal discovery of aerobic glycolysis established metabolic reprogramming as one of the first distinguishing characteristics of cancer¹. The field of cancer metabolism subsequently revealed additional metabolic alterations in cancer by focusing on central carbon metabolism, including the citric acid cycle and pentose phosphate pathway. Recent reports have, however, uncovered substantial non-carbon metabolism contributions to cancer cell viability and growth. Amino acids, nutrients vital to the survival of all cell types, experience reprogrammed metabolism in cancer. This review outlines the diverse roles of amino acids within the tumor and in the tumor microenvironment. Beyond their role in biosynthesis, they serve as energy sources and help maintain redox balance. In addition, amino acid derivatives contribute to epigenetic regulation and immune responses linked to tumorigenesis and metastasis. Furthermore, in discussing the transporters and transaminases that mediate amino acid uptake and synthesis, we identify potential metabolic liabilities as targets for therapeutic intervention.

Cholestatic liver disease results increased production of reactive aldehydes and an atypical periportal hepatic antioxidant response

Cholangiopathies such as primary sclerosing cholangitis (PSC) are chronic liver diseases characterized by increased cholestasis, biliary inflammation and oxidative stress. The objective of this study was to elucidate the impact of cholestatic injury on oxidative stress-related factors. Using hepatic tissue and whole cell liver extracts (LE) isolated from 11-week old C57BL/6J (WT) and Mdr2^KO mice, inflammation and oxidative stress was assessed. Concurrently, specific targets of carbonylation were assessed in LE prepared from murine groups as well as from normal and human patients with end-stage PSC. Identified carbonylated proteins were further evaluated using bioinformatics analyses. Picrosirius red staining revealed extensive fibrosis in Mdr2^KO liver, and fibrosis colocalized with increased periportal inflammatory cells and both acrolein and 4-HNE staining. Western blot analysis revealed elevated periportal expression of antioxidant proteins Cbr3, GSTμ, Prdx5, TrxR1 and HO-1 but not GCLC, GSTπ or catalase in the Mdr2^KO group when compared to WT. From immunohistochemical analysis, increased periportal reactive aldehyde production colocalized with elevated staining of Cbr3, GSTμ and TrxR1 but surprisingly not with Nrf2. Mass spectrometric analysis revealed an increase in carbonylated proteins in the Mdr2^KO and PSC groups compared to respective controls. Gene ontology and KEGG pathway analysis of carbonylated proteins revealed a propensity for increased carbonylation of proteins broadly involved in metabolic processes as well more specifically in Rab-mediated signal transduction, lysosomes and the large ribosomal subunit in human PSC. Western blot analysis of Rab-GTPase expression revealed no significant differences in Mdr2^KO mice when compared to WT livers. In contrast, PSC tissue exhibited decreased levels of Rabs 4, 5 and increased abundance of Rabs 6 and 9a protein. Results herein reveal that cholestasis induces stage-dependent increases in periportal oxidative stress responses and protein carbonylation, potentially contributing to pathogenesis in Mdr2^KO. Furthermore, during early stage cholestasis, there is cell-specific upregulation of some but not all, antioxidant proteins.

Exploring cysteine regulation in cancer cell survival with a highly specific "Lock and Key" fluorescent probe for cysteine

Using a highly specific “lock and key” fluorescent Cys probe, we confirmed that targeting Cys metabolism to deplete intracellular Cys is a more potent strategy to sensitize cancer cells to chemotherapies.

To probe the regulatory roles of cysteine (Cys) in cancer cell survival, a highly selective and sensitive fluorescent Cys probe SiR was developed by employing a novel “lock and key” strategy, which allows Cys to be detected without any interference or probe consumption caused by the intracellular high concentration of glutathione (GSH). Using SiR, we confirmed that inhibiting cystine (Cys₂) transporter system x_c^– to deplete intracellular Cys is more efficient than inhibiting glutamate–cysteine ligase GCL to deplete intracellular GSH for sensitizing cancer cells to chemotherapy. Moreover, with the probe, a possible self-protection mechanism of cancer cells was indicated: when extracellular Cys sources are blocked, cancer cells could still survive by multidrug resistance protein transporter (Mrp1)-mediated export of intracellular GSH/GSSG as sources to supply intracellular Cys for resisting detrimental oxidative stress. Based on this finding, we further confirmed that abrogating the self-protection mechanism is an even more efficient strategy for sensitizing cancer cells to chemotherapy.

The effect of short-term methionine restriction on glutathione synthetic capacity and antioxidant responses at the whole tissue and mitochondrial level in the rat liver

Dietary methionine restriction (MR) where methionine is the sole source of sulfur amino acid increases lifespan in diverse species. Methionine restricted rodents experience a decrease in glutathione (GSH), a major antioxidant, in several tissues, which is paradoxical to longevity interventions because tissues with low GSH might experience more oxidative damage. Liver plays a key role in GSH synthesis and here we examined how MR influences GSH metabolism in the liver. We also hypothesised that low GSH might be subsidized by compensatory pathway(s) in the liver. To investigate GSH synthesis and antioxidant responses, Fischer-344 rats were given either a MR diet or a control diet for 8 weeks. Based on γ-glutamylcysteine synthetase activity, GSH synthetic capacity did not respond to low dietary methionine availability. Tissue level protein and lipid oxidation markers do not support elevated oxidative damage, despite low GSH availability. Whole tissue and mitochondrial level responses to MR differed. Specifically, the activity of glutathione reductase and thioredoxin reductase increase in whole liver tissue which might offset the effects of declined GSH availability whereas mitochondrial GSH levels were unperturbed by MR. Moreover, enhanced proton leak in liver mitochondria by MR (4 week) presumably diminishes ROS production. Taken together, we suggest that the effect of low GSH in liver tissue is subsidized, at least in part, by increased antioxidant activity and possibly by enhanced mitochondrial proton leak.

Targeting Ferroptosis to Iron Out Cancer

One of the key challenges in cancer research is how to effectively kill cancer cells while leaving the healthy cells intact. Cancer cells often have defects in cell death executioner mechanisms, which is one of the main reasons for therapy resistance. To enable growth, cancer cells exhibit an increased iron demand compared with normal, non-cancer cells. This iron dependency can make cancer cells more vulnerable to iron-catalyzed necrosis, referred to as ferroptosis. The identification of FDA-approved drugs as ferroptosis inducers creates high expectations for the potential of ferroptosis to be a new promising way to kill therapy-resistant cancers.

GPX4 at the Crossroads of Lipid Homeostasis and Ferroptosis

Oxygen is necessary for aerobic metabolism but can cause the harmful oxidation of lipids and other macromolecules. Oxidation of cholesterol and phospholipids containing polyunsaturated fatty acyl chains can lead to lipid peroxidation, membrane damage, and cell death. Lipid hydroperoxides are key intermediates in the process of lipid peroxidation. The lipid hydroperoxidase glutathione peroxidase 4 (GPX4) converts lipid hydroperoxides to lipid alcohols, and this process prevents the iron (Fe²⁺ )-dependent formation of toxic lipid reactive oxygen species (ROS). Inhibition of GPX4 function leads to lipid peroxidation and can result in the induction of ferroptosis, an iron-dependent, non-apoptotic form of cell death. This review describes the formation of reactive lipid species, the function of GPX4 in preventing oxidative lipid damage, and the link between GPX4 dysfunction, lipid oxidation, and the induction of ferroptosis.

Role of GPX4 in ferroptosis and its pharmacological implication

Ferroptosis is a non-apoptotic form of cell death characterized by iron-dependent lipid peroxidation and metabolic constraints. Dependence on NADPH/H⁺, polyunsaturated fatty acid metabolism, and the mevalonate and glutaminolysis metabolic pathways have been implicated in this novel form of regulated necrotic cell death. Genetic studies performed in cells and mice established the selenoenzyme glutathione peroxidase (GPX4) as the key regulator of this form of cell death. Besides these genetic models, the identification of a series of small molecule ferroptosis-specific inhibitors and inducers have not only helped in the delineation of the molecular underpinnings of ferroptosis but they might also prove highly beneficial when tipping the balance between cell death inhibition and induction in the context of degenerative diseases and cancer, respectively. In the latter, the recent recognition that a subset of cancer cell lines including certain triple negative breast cancer cells and those of therapy-resistant high-mesenchymal cell state present a high dependence on this lipid make-up offers unprecedented opportunities to eradicate difficult to treat cancers. Due to the rapidly growing interest in this form of cell death, we provide an overview herein what we know about this field today and its future translational impact.

The development of the concept of ferroptosis

The term ferroptosis was coined in 2012 to describe an iron-dependent regulated form of cell death caused by the accumulation of lipid-based reactive oxygen species; this type of cell death was found to have molecular characteristics distinct from other forms of regulated cell death. Features of ferroptosis have been observed periodically over the last several decades, but these molecular features were not recognized as evidence of a distinct form of cell death until recently. Here, we describe the history of observations consistent with the current definition of ferroptosis, as well as the advances that contributed to the emergence of the concept of ferroptosis. We also discuss recent implications and applications of manipulations of the ferroptotic death pathway.

Assessment of Tumor Redox Status through (S)-4-(3-[18F]fluoropropyl)-L-Glutamic Acid PET Imaging of System xc - Activity

The cell's endogenous antioxidant system is vital to maintenance of redox homeostasis. Despite its central role in normal and pathophysiology, no noninvasive tools exist to measure this system in patients. The cystine/glutamate antiporter system xc - maintains the balance between intracellular reactive oxygen species and antioxidant production through the provision of cystine, a key precursor in glutathione biosynthesis. Here, we show that tumor cell retention of a system xc --specific PET radiotracer, (S)-4-(3-[18F]fluoropropyl)-L-glutamic acid ([18F]FSPG), decreases in proportion to levels of oxidative stress following treatment with a range of redox-active compounds. The decrease in [18F]FSPG retention correlated with a depletion of intracellular cystine resulting from increased de novo glutathione biosynthesis, shown through [U-13C6, U-15N2]cystine isotopic tracing. In vivo, treatment with the chemotherapeutic doxorubicin decreased [18F]FSPG tumor uptake in a mouse model of ovarian cancer, coinciding with markers of oxidative stress but preceding tumor shrinkage and decreased glucose utilization. Having already been used in pilot clinical trials, [18F]FSPG PET could be rapidly translated to the clinic as an early redox indicator of tumor response to treatment. SIGNIFICANCE: [18F]FSPG PET imaging provides a sensitive noninvasive measure of tumor redox status and provides an early marker of tumor response to therapy.See related commentary by Lee et al., p. 701.

Ferroptosis: The Greasy Side of Cell Death

Ferroptosis is a form of cell death that requires phospholipid peroxidation and has attracted increased attention, both as a means to eradicate tumors resistant to standard chemotherapy and for its potential contribution to tissue damage such as in ischemia/reperfusion. The center stage taken by phospholipid peroxidation in ferroptosis is highlighted by recent discoveries that demonstrate an intricate regulation of both the metabolism of polyunsaturated fatty acids as well as mechanisms leading to their oxidation. These metabolic steps converge at the point of ferroptosis execution through mechanisms that are now only starting to be understood. In this short review, we provide an appraisal of some of the recent advances in the understanding of the ferroptosis process and also provide some perspectives of where this knowledge could take us.

Ferroptosis and necroinflammation, a yet poorly explored link

Ferroptosis is a non-apoptotic form of cell death characterized by overwhelming iron-dependent lipid peroxidation, which contributes to a number of pathologies, most notably tissue ischemia/reperfusion injury, neurodegeneration and cancer. Cysteine availability, glutathione biosynthesis, polyunsaturated fatty acid metabolism and modulation of the phospholipidome are the key events of this necrotic cell death pathway. Non-enzymatic and enzymatic lipoxygenase (LOX)-mediated lipid peroxidation of lipid bilayers is efficiently counteracted by the glutathione (GSH)/glutathione peroxidase 4 (GPX4) axis. Preliminary studies suggest that bursting ferroptotic cells release pro-inflammatory damage-associated molecular patterns (DAMPs) that trigger the innate immune system as exemplified by diseased kidney and brain tissues where ferroptosis contributes to organ demise in a predominant manner. The GSH/GPX4 node is known to control the activities of LOX and prostaglandin-endoperoxide synthase (PTGS) via the so-called peroxide tone. Since LOX and PTGS products do have pro- and anti-inflammatory effects, one may speculate that these enzymes contribute to the ferroptotic process on several levels in cell-autonomous and non-autonomous ways. Hence, this review provides the reader with an outline on what is currently known about the link between ferroptosis and necroinflammation and discusses critical events that may alert the innate immune system in early phases when cells become sensitized towards ferroptosis.

S-Alk(en)ylmercaptocysteine suppresses LPS-induced pro-inflammatory responses in murine macrophages through inhibition of NF-κB pathway and modulation of thiol redox status

The Allium vegetable-derived metabolite, S-alk(en)ylmercaptocysteine (CySSR), has been reported to modulate oxidative stress and inflammatory responses. However, the underlying mechanisms of action and structure-activity relationships are not completely understood. We investigated the mechanistic basis of the protective effects of CySSR on pro-inflammatory responses involving redox/oxidative stress induced by E. coli lipopolysaccharide (LPS) using RAW 264.7 cells. CySSR (R = allyl, "A" or 1-propenyl, "Pe") pre-treatments conferred concentration-dependent reductions in cytokines (TNF-α, IL-1β and IL-6), NO production and iNOS (inducible nitric synthase) overexpression, and attenuated oxidant production in LPS-stimulated RAW 264.7 cells where viability remained > 90%. These protective effects were manifested through inhibited activation of the nuclear factor-kappa B (NF-κB) signaling pathway via suppression of the IκB kinases (IKK) phosphorylation possibly by transforming growth factor β-activated kinase 1 or a kinase further upstream the canonical NF-κB signaling pathway. The attenuation of LPS-induced inflammation by CySSRs was associated with enhanced levels of cellular cysteine (CySH) and glutathione (GSH) mediated by cellular import/reduction of CySSR and the induction of glutamate cysteine ligase (GCL), one of > 200 nuclear factor erythroid 2-related factor 2 (Nrf2) regulated proteins. The reduction of anti-inflammatory effect of CySSR following pretreatment of cells with L-buthionine-S,R-sulfoximine (BSO) implicates GSH having a major role in reducing inflammation, likely in the context of other Nrf2-regulated antioxidant enzymes that scavenge H₂O₂ and peroxides using GSH as co-substrate. The anti-inflammatory effect of CySSPe was significantly greater than CySSA for almost all indicators measured, and cell metabolites of CySSRs may have a role in attenuating NF-κB signaling.

Golgi stress mediates redox imbalance and ferroptosis in human cells

Cytotoxic activities of several Golgi-dispersing compounds including AMF-26/M-COPA, brefeldin A and golgicide A have previously been shown to induce autophagy or apoptosis. Here, we demonstrate that these Golgi disruptors also trigger ferroptosis, a non-apoptotic form of cell death characterized by iron-dependent oxidative degradation of lipids. Inhibitors of ferroptosis not only counteract cell death, but they also protect from Golgi dispersal and inhibition of protein secretion in response to several Golgi stress agents. Furthermore, the application of sublethal doses of ferroptosis-inducers such as erastin and sorafenib, low cystine growth conditions, or genetic knockdown of SLC7A11 and GPX4 all similarly protect cells from Golgi stress and lead to modulation of ACSL4, SLC7A5, SLC7A11 or GPX4 levels. Collectively, this study suggests a previously unrecognized function of the Golgi apparatus, which involves cellular redox control and prevents ferroptotic cell death.

Hamed Alborzinia et al. show that Golgi-dispersing compounds trigger iron-dependent oxidative degradation of lipids, inducing a non-apoptotic cell death called ferroptosis. This study provides insight into the role of Golgi apparatus for preventing ferroptotic cell death through its cellular redox control.

CD44v9 is associated with epithelial-mesenchymal transition and poor outcomes in esophageal squamous cell carcinoma

CD44 serves as a marker of cancer stem cells. Alternative splicing generates the CD44v9 isoform. Cancer stem cells are associated with the epithelial-mesenchymal transition in cancers, although little is known about their role in esophageal squamous cell carcinoma. Here, we aimed to clarify the relationship between CD44v9 expression, the epithelial-mesenchymal transition, and clinicopathological features of patients with esophageal squamous cell carcinoma. CD44v9 levels were higher at the tumor invasive front compared with the center of the tumor and higher in metastatic lymph nodes compared with primary tumors. High levels of CD44v9 at the tumor invasive front were significantly associated with deeper tumor invasion and shorter overall survival and recurrence-free survival. The expression of CD44v9 was increased by treatment with transforming growth factor-β, which induced esophageal squamous cell carcinoma cells to undergo the epithelial-mesenchymal transition. Moreover, inhibition of CD44v9 expression decreased the migration and invasiveness of esophageal squamous cell carcinoma cells. These results indicate that the expression of CD44v9 at the tumor invasive front induced by stemness was strongly associated with the epithelial-mesenchymal transition and poor prognosis of patients with esophageal squamous cell carcinoma. CD44v9 may therefore serve as a novel prognostic biomarker and a potential therapeutic target for esophageal squamous cell carcinoma.

Disentangling associations between DNA methylation and blood lipids: a Mendelian randomization approach

BACKGROUND

DNA methylation is an epigenetic mechanism that has been proposed as a possible link between genetic and environmental determinants of disease. Prior studies reported robust associations between the methylation of specific cytosine-phosphate-guanine (CpG) sites and plasma lipids, namely triglycerides (TGs) and high-density lipoprotein cholesterol (HDL-C). However, the causality of the observed association remains elusive, hampered by weak instrumental variables for methylation status.

AIM

We present a novel application of the elastic net approach to implement a bidirectional Mendelian randomization approach to inferring causal relationships between candidate CpGs and plasma lipids in GAW20 data.

METHODS

We used DNA methylation, TGs, and HDL-C measured during the visit 2. Based on prior findings, we selected 5 methylation markers (cg00574958, cg07504977, cg06690548, cg19693031, and cg03717755) related to TGs, 2 markers (cg09572125 and cg02650017) related to HDL-C, and 2 markers (cg06500161 and cg11024682) related to both traits. We implemented an elastic net approach to improve the selection of the genetic instrument for the methylation markers, followed by bidirectional Mendelian randomization 2-stage least-squares regression.

RESULTS

We observed causal effects of blood fasting TGs on the methylation levels of cg00574958 (CPT1A) and cg06690548 (SLC7A11). For cg00574958, our findings were also consistent with the reverse direction of association, that is, from CPT1A methylation to TGs.

CONCLUSIONS

Current evidence does not rule out either direction of association between the methylation of the cg00574958 CPT1A locus and plasma TGs, highlighting the complexity of lipid homeostasis. We also demonstrated a novel approach to improve instrument selection in DNA methylation studies.

NMDA-receptor inhibition and oxidative stress during hippocampal maturation differentially alter parvalbumin expression and gamma-band activity

Dysfunction of parvalbumin (PV)-expressing interneurons is thought to underlie the alterations of gamma-band oscillations observed in schizophrenia. Although the pathomechanisms of this disease remain unclear, oxidative stress induced by NMDA receptor (NMDAR) hypofunction and decreased glutathione (GSH) synthesizing capacity have been shown to lead to PV-loss and aberrant oscillatory activity. However, the individual contributions of NMDAR-inhibition and GSH-depletion to the developmental alterations observed in schizophrenia are largely unknown. We therefore investigated each condition in isolation using hippocampal slice cultures wherein interneuron maturation occurs entirely in vitro. Although both treatments caused oxidative stress, NMDAR-inhibition led to an immediate reduction in gamma oscillation frequency and a delayed loss of PV. In contrast, GSH-depletion immediately decreased PV expression and increased power, without affecting frequency. Hence, although disturbances of PV-expression and gamma oscillations coexist in schizophrenia, they can arise from separate pathological processes.

Oxytosis/Ferroptosis-(Re-) Emerging Roles for Oxidative Stress-Dependent Non-apoptotic Cell Death in Diseases of the Central Nervous System

Although nerve cell death is the hallmark of many neurological diseases, the processes underlying this death are still poorly defined. However, there is a general consensus that neuronal cell death predominantly proceeds by regulated processes. Almost 30 years ago, a cell death pathway eventually named oxytosis was described in neuronal cells that involved glutathione depletion, reactive oxygen species production, lipoxygenase activation, and calcium influx. More recently, a cell death pathway that involved many of the same steps was described in tumor cells and termed ferroptosis due to a dependence on iron. Since then there has been a great deal of discussion in the literature about whether these are two distinct pathways or cell type- and insult-dependent variations on the same pathway. In this review, we compare and contrast in detail the commonalities and distinctions between the two pathways concluding that the molecular pathways involved in the regulation of ferroptosis and oxytosis are highly similar if not identical. Thus, we suggest that oxytosis and ferroptosis should be regarded as two names for the same cell death pathway. In addition, we describe the potential physiological relevance of oxytosis/ferroptosis in multiple neurological diseases.

Cysteine Metabolism in Neuronal Redox Homeostasis

Besides its essential role in protein synthesis, cysteine plays vital roles in redox homeostasis, being a component of the major antioxidant glutathione (GSH) and a potent antioxidant by itself. In addition, cysteine undergoes a variety of post-translational modifications that modulate several physiological processes. It is becoming increasingly clear that redox-modulated events play important roles not only in peripheral tissues but also in the brain where cysteine disposition is central to these pathways. Dysregulated cysteine metabolism is associated with several neurodegenerative disorders. Accordingly, restoration of cysteine balance has therapeutic benefits. This review discusses metabolic signaling pathways pertaining to cysteine disposition in the brain under normal and pathological conditions, highlighting recent findings on cysteine metabolism during aging and in neurodegenerative conditions such as Huntington's disease (HD) and molybdenum cofactor (MoCo) deficiency (MoCD) among others.

Irreversible inhibition of cytosolic thioredoxin reductase 1 as a mechanistic basis for anticancer therapy

Cancer cells adapt to their inherently increased oxidative stress through activation of the glutathione (GSH) and thioredoxin (TXN) systems. Inhibition of both of these systems effectively kills cancer cells, but such broad inhibition of antioxidant activity also kills normal cells, which is highly unwanted in a clinical setting. We therefore evaluated targeting of the TXN pathway alone and, more specifically, selective inhibition of the cytosolic selenocysteine-containing enzyme TXN reductase 1 (TXNRD1). TXNRD1 inhibitors were discovered in a large screening effort and displayed increased specificity compared to pan-TXNRD inhibitors, such as auranofin, that also inhibit the mitochondrial enzyme TXNRD2 and additional targets. For our lead compounds, TXNRD1 inhibition correlated with cancer cell cytotoxicity, and inhibitor-triggered conversion of TXNRD1 from an antioxidant to a pro-oxidant enzyme correlated with corresponding increases in cellular production of H2O2 In mice, the most specific TXNRD1 inhibitor, here described as TXNRD1 inhibitor 1 (TRi-1), impaired growth and viability of human tumor xenografts and syngeneic mouse tumors while having little mitochondrial toxicity and being better tolerated than auranofin. These results display the therapeutic anticancer potential of irreversibly targeting cytosolic TXNRD1 using small molecules and present potent and selective TXNRD1 inhibitors. Given the pronounced up-regulation of TXNRD1 in several metastatic malignancies, it seems worthwhile to further explore the potential benefit of specific irreversible TXNRD1 inhibitors for anticancer therapy.

The ferroptosis inducer erastin irreversibly inhibits system xc- and synergizes with cisplatin to increase cisplatin's cytotoxicity in cancer cells

System x_c⁻ was recently described as the most upstream node in a novel form of regulated necrotic cell death, called ferroptosis. In this context, the small molecule erastin was reported to target and inhibit system x_c⁻, leading to cysteine starvation, glutathione depletion and consequently ferroptotic cell death. Although the inhibitory effect of erastin towards system x_c⁻ is well-documented, nothing is known about its mechanism of action. Therefore, we sought to interrogate in more detail the underlying mechanism of erastin’s pro-ferroptotic effects. When comparing with some well-known inhibitors of system x_c⁻, erastin was the most efficient inhibitor acting at low micromolar concentrations. Notably, only a very short exposure of cells with low erastin concentrations was sufficient to cause a strong and persistent inhibition of system x_c⁻, causing glutathione depletion. These inhibitory effects towards system x_c⁻ did not involve cysteine modifications of the transporter. More importantly, short exposure of tumor cells with erastin strongly potentiated the cytotoxic effects of cisplatin to efficiently eradicate tumor cells. Hence, our data suggests that only a very short pre-treatment of erastin suffices to synergize with cisplatin to efficiently induce cancer cell death, findings that might guide us in the design of novel cancer treatment paradigms.

Insulin resistance, an unmasked culprit in depressive disorders: Promises for interventions

Depressive disorders constitute a set of debilitating diseases with psychological, societal, economic and humanitarian consequences for millions of people worldwide. Scientists are beginning to understand the reciprocal communication between the brain and the rest of the body in the etiology of these diseases. In particular, scientists have noted a connection between depressive disorders, which are primarily seen as brain-based, and, insulin resistance (IR), a modifiable metabolic inflammatory state that is typically seen as peripheral. We highlight evidence showing how treating IR, with drugs or behavioral interventions, may ameliorate or possibly prevent, depressive disorders and their long-term consequences at various stages of the life course. This article is part of the Special Issue entitled 'Metabolic Impairment as Risk Factors for Neurodegenerative Disorders.'

Multifaceted Roles of Glutathione and Glutathione-Based Systems in Carcinogenesis and Anticancer Drug Resistance

SIGNIFICANCE

Glutathione is the most abundant antioxidant molecule in living organisms and has multiple functions. Intracellular glutathione homeostasis, through its synthesis, consumption, and degradation, is an intricately balanced process. Glutathione levels are often high in tumor cells before treatment, and there is a strong correlation between elevated levels of intracellular glutathione/sustained glutathione-mediated redox activity and resistance to pro-oxidant anticancer therapy. Recent Advances: Ample evidence demonstrates that glutathione and glutathione-based systems are particularly relevant in cancer initiation, progression, and the development of anticancer drug resistance.

CRITICAL ISSUES

This review highlights the multifaceted roles of glutathione and glutathione-based systems in carcinogenesis, anticancer drug resistance, and clinical applications.

FUTURE DIRECTIONS

The evidence summarized here underscores the important role played by glutathione and the glutathione-based systems in carcinogenesis and anticancer drug resistance. Future studies should address mechanistic questions regarding the distinct roles of glutathione in different stages of cancer development and cancer cell death. It will be important to study how metabolic alterations in cancer cells can influence glutathione homeostasis. Sensitive approaches to monitor glutathione dynamics in subcellular compartments will be an indispensible step. Therapeutic perspectives should focus on mechanism-based rational drug combinations that are directed against multiple redox targets using effective, specific, and clinically safe inhibitors. This new strategy is expected to produce a synergistic effect, prevent drug resistance, and diminish doses of single drugs. Antioxid. Redox Signal. 27, 1217-1234.

Oncogene-Selective Sensitivity to Synchronous Cell Death following Modulation of the Amino Acid Nutrient Cystine

Cancer cells reprogram their metabolism, altering both uptake and utilization of extracellular nutrients. We individually depleted amino acid nutrients from isogenic cells expressing commonly activated oncogenes to identify correspondences between nutrient supply and viability. In HME (human mammary epithelial) cells, deprivation of cystine led to increased cell death in cells expressing an activated epidermal growth factor receptor (EGFR) mutant. Cell death occurred via synchronous ferroptosis, with generation of reactive oxygen species (ROS). Hydrogen peroxide promoted cell death, as both catalase and inhibition of NADPH oxidase 4 (NOX4) blocked ferroptosis. Blockade of EGFR or mitogen-activated protein kinase (MAPK) signaling similarly protected cells from ferroptosis, whereas treatment of xenografts derived from EGFR mutant non-small-cell lung cancer (NSCLC) with a cystine-depleting enzyme inhibited tumor growth in mice. Collectively, our results identify a potentially exploitable sensitization of some EGFR/MAPK-driven tumors to ferroptosis following cystine depletion.

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A nutrient depletion screen revealed a selective role for cystine in promoting viability

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Cystine was shown to promote viability by preventing ferroptosis

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Sensitivity to depletion of cystine was related to activation of MAPK

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Depletion of cystine inhibited tumor growth in a NSCLC xenograft model

Cystine uptake through the cystine/glutamate antiporter xCT triggers glioblastoma cell death under glucose deprivation

Oncogenic signaling in cancer cells alters glucose uptake and utilization to supply sufficient energy and biosynthetic intermediates for survival and sustained proliferation. Oncogenic signaling also prevents oxidative stress and cell death caused by increased production of reactive oxygen species. However, elevated glucose metabolism in cancer cells, especially in glioblastoma, results in the cells becoming sensitive to glucose deprivation (i.e. in high glucose dependence), which rapidly induces cell death. However, the precise mechanism of this type of cell death remains unknown. Here, we report that glucose deprivation alone does not trigger glioblastoma cell death. We found that, for cell death to occur in glucose-deprived glioblastoma cells, cystine and glutamine also need to be present in culture media. We observed that cystine uptake through the cystine/glutamate antiporter xCT under glucose deprivation rapidly induces NADPH depletion, reactive oxygen species accumulation, and cell death. We conclude that although cystine uptake is crucial for production of antioxidant glutathione in cancer cells its transport through xCT also induces oxidative stress and cell death in glucose-deprived glioblastoma cells. Combining inhibitors targeting cancer-specific glucose metabolism with cystine and glutamine treatment may offer a therapeutic approach for glioblastoma tumors exhibiting high xCT expression.

Targeting redox homeostasis in rhabdomyosarcoma cells: GSH-depleting agents enhance auranofin-induced cell death

Rhabdomyosarcoma (RMS) cells have recently been reported to be sensitive to oxidative stress. Therefore, we investigated whether concomitant inhibition of the two main antioxidant defense pathways, that is, the thioredoxin (TRX) and the glutathione (GSH) systems, presents a new strategy to trigger cell death in RMS. In this study, we discover that GSH-depleting agents, i.e. γ-glutamylcysteine synthetase inhibitor, buthionine sulfoximine (BSO) or the cystine/glutamate antiporter inhibitor erastin (ERA), synergize with thioredoxin reductase (TrxR) inhibitor auranofin (AUR) to induce cell death in RMS cells. Interestingly, AUR causes accumulation of ubiquitinated proteins when combined with BSO or ERA, in line with recent reports showing that AUR inhibits the proteasome besides TrxR. Consistently, AUR/BSO or AUR/ERA cotreatment increases ubiquitination and expression of the short-lived proteins NOXA and MCL-1, accompanied by increased binding of NOXA to MCL-1. Notably, NOXA knockdown significantly rescues RMS cells from AUR/BSO- or AUR/ERA-induced cell death. In addition, AUR acts together with BSO or ERA to stimulate BAX/BAK and caspase activation. Of note, BSO or ERA abolish the AUR-stimulated increase in GSH levels, leading to reduced GSH levels upon cotreatment. Although AUR/BSO or AUR/ERA cotreatment enhances reactive oxygen species (ROS) production, only thiol-containing antioxidants (i.e., N-acetylcysteine (NAC), GSH), but not the non-thiol-containing ROS scavenger α-Tocopherol consistently suppress AUR/BSO- and AUR/ERA-stimulated cell death in both cell lines. Importantly, re-supply of GSH or its precursor NAC completely prevents AUR/ERA- and AUR/BSO-induced accumulation of ubiquitinated proteins, NOXA upregulation and cell death, indicating that GSH depletion rather than ROS production is critical for AUR/BSO- or AUR/ERA-mediated cell death. Thus, by demonstrating that GSH-depleting agents enhance the antitumor activity of AUR, we highlight new treatment options for RMS by targeting the redox homeostasis.

β1-Integrin Deletion From the Lens Activates Cellular Stress Responses Leading to Apoptosis and Fibrosis

Purpose

Previous research showed that the absence of β1-integrin from the mouse lens after embryonic day (E) 13.5 (β1MLR10) leads to the perinatal apoptosis of lens epithelial cells (LECs) resulting in severe microphthalmia. This study focuses on elucidating the molecular connections between β1-integrin deletion and this phenotype.

Methods

RNA sequencing was performed to identify differentially regulated genes (DRGs) in β1MLR10 lenses at E15.5. By using bioinformatics analysis and literature searching, Egr1 (early growth response 1) was selected for further study. The activation status of certain signaling pathways (focal adhesion kinase [FAK]/Erk, TGF-β, and Akt signaling) was studied via Western blot and immunohistochemistry. Mice lacking both β1-integrin and Egr1 genes from the lenses were created (β1MLR10/Egr1-/-) to study their relationship.

Results

RNA sequencing identified 120 DRGs that include candidates involved in the cellular stress response, fibrosis, and/or apoptosis. Egr1 was investigated in detail, as it mediates cellular stress responses in various cell types, and is recognized as an upstream regulator of numerous other β1MLR10 lens DRGs. In β1MLR10 mice, Egr1 levels are elevated shortly after β1-integrin loss from the lens. Further, pErk1/2 and pAkt are elevated in β1MLR10 LECs, thus providing the potential signaling mechanism that causes Egr1 upregulation in the mutant. Indeed, deletion of Egr1 from β1MLR10 lenses partially rescues the microphthalmia phenotype.

Conclusions

β1-integrin regulates the appropriate levels of Erk1/2 and Akt phosphorylation in LECs, whereas its deficiency results in the overexpression of Egr1, culminating in reduced cell survival. These findings provide insight into the molecular mechanism underlying the microphthalmia observed in β1MLR10 mice.

New Challenges to Study Heterogeneity in Cancer Redox Metabolism

Reactive oxygen species (ROS) are important pathophysiological molecules involved in vital cellular processes. They are extremely harmful at high concentrations because they promote the generation of radicals and the oxidation of lipids, proteins, and nucleic acids, which can result in apoptosis. An imbalance of ROS and a disturbance of redox homeostasis are now recognized as a hallmark of complex diseases. Considering that ROS levels are significantly increased in cancer cells due to mitochondrial dysfunction, ROS metabolism has been targeted for the development of efficient treatment strategies, and antioxidants are used as potential chemotherapeutic drugs. However, initial ROS-focused clinical trials in which antioxidants were supplemented to patients provided inconsistent results, i.e., improved treatment or increased malignancy. These different outcomes may result from the highly heterogeneous redox responses of tumors in different patients. Hence, population-based treatment strategies are unsuitable and patient-tailored therapeutic approaches are required for the effective treatment of patients. Moreover, due to the crosstalk between ROS, reducing equivalents [e.g., NAD(P)H] and central metabolism, which is heterogeneous in cancer, finding the best therapeutic target requires the consideration of system-wide approaches that are capable of capturing the complex alterations observed in all of the associated pathways. Systems biology and engineering approaches may be employed to overcome these challenges, together with tools developed in personalized medicine. However, ROS- and redox-based therapies have yet to be addressed by these methodologies in the context of disease treatment. Here, we review the role of ROS and their coupled redox partners in tumorigenesis. Specifically, we highlight some of the challenges in understanding the role of hydrogen peroxide (H₂O₂), one of the most important ROS in pathophysiology in the progression of cancer. We also discuss its interplay with antioxidant defenses, such as the coupled peroxiredoxin/thioredoxin and glutathione/glutathione peroxidase systems, and its reducing equivalent metabolism. Finally, we highlight the need for system-level and patient-tailored approaches to clarify the roles of these systems and identify therapeutic targets through the use of the tools developed in personalized medicine.

On the Mechanism of Cytoprotection by Ferrostatin-1 and Liproxstatin-1 and the Role of Lipid Peroxidation in Ferroptotic Cell Death

Ferroptosis is a form of regulated necrosis associated with the iron-dependent accumulation of lipid hydroperoxides that may play a key role in the pathogenesis of degenerative diseases in which lipid peroxidation has been implicated. High-throughput screening efforts have identified ferrostatin-1 (Fer-1) and liproxstatin-1 (Lip-1) as potent inhibitors of ferroptosis - an activity that has been ascribed to their ability to slow the accumulation of lipid hydroperoxides. Herein we demonstrate that this activity likely derives from their reactivity as radical-trapping antioxidants (RTAs) rather than their potency as inhibitors of lipoxygenases. Although inhibited autoxidations of styrene revealed that Fer-1 and Lip-1 react roughly 10-fold more slowly with peroxyl radicals than reactions of α-tocopherol (α-TOH), they were significantly more reactive than α-TOH in phosphatidylcholine lipid bilayers - consistent with the greater potency of Fer-1 and Lip-1 relative to α-TOH as inhibitors of ferroptosis. None of Fer-1, Lip-1, and α-TOH inhibited human 15-lipoxygenase-1 (15-LOX-1) overexpressed in HEK-293 cells when assayed at concentrations where they inhibited ferroptosis. These results stand in stark contrast to those obtained with a known 15-LOX-1 inhibitor (PD146176), which was able to inhibit the enzyme at concentrations where it was effective in inhibiting ferroptosis. Given the likelihood that Fer-1 and Lip-1 subvert ferroptosis by inhibiting lipid peroxidation as RTAs, we evaluated the antiferroptotic potential of 1,8-tetrahydronaphthyridinols (hereafter THNs): rationally designed radical-trapping antioxidants of unparalleled reactivity. We show for the first time that the inherent reactivity of the THNs translates to cell culture, where lipophilic THNs were similarly effective to Fer-1 and Lip-1 at subverting ferroptosis induced by either pharmacological or genetic inhibition of the hydroperoxide-detoxifying enzyme Gpx4 in mouse fibroblasts, and glutamate-induced death of mouse hippocampal cells. These results demonstrate that potent RTAs subvert ferroptosis and suggest that lipid peroxidation (autoxidation) may play a central role in the process.

Iron and ferroptosis: A still ill-defined liaison

Ferroptosis is a recently described form of regulated necrotic cell death, which appears to contribute to a number of diseases, such as tissue ischemia/reperfusion injury, acute renal failure, and neurodegeneration. A hallmark of ferroptosis is iron-dependent lipid peroxidation, which can be inhibited by the key ferroptosis regulator glutathione peroxidase 4(Gpx4), radical trapping antioxidants and ferroptosis-specific inhibitors, such as ferrostatins and liproxstatins, as well as iron chelation. Although great strides have been made towards a better understanding of the proximate signals of distinctive lipid peroxides in ferroptosis, still little is known about the mechanistic implication of iron in the ferroptotic process. Hence, this review aims at summarizing recent advances in our understanding to what is known about enzymatic and nonenzymatic routes of lipid peroxidation, the involvement of iron in this process and the identification of novel players in ferroptotic cell death. Additionally, we review early works carried out long time before the term "ferroptosis" was actually introduced but which were instrumental in a better understanding of the role of ferroptosis in physiological and pathophysiological contexts. © 2017 IUBMB Life, 69(6):423-434, 2017.

The Architecture of Thiol Antioxidant Systems among Invertebrate Parasites

The use of oxygen as the final electron acceptor in aerobic organisms results in an improvement in the energy metabolism. However, as a byproduct of the aerobic metabolism, reactive oxygen species are produced, leaving to the potential risk of an oxidative stress. To contend with such harmful compounds, living organisms have evolved antioxidant strategies. In this sense, the thiol-dependent antioxidant defense systems play a central role. In all cases, cysteine constitutes the major building block on which such systems are constructed, being present in redox substrates such as glutathione, thioredoxin, and trypanothione, as well as at the catalytic site of a variety of reductases and peroxidases. In some cases, the related selenocysteine was incorporated at selected proteins. In invertebrate parasites, antioxidant systems have evolved in a diversity of both substrates and enzymes, representing a potential area in the design of anti-parasite strategies. The present review focus on the organization of the thiol-based antioxidant systems in invertebrate parasites. Differences between these taxa and its final mammal host is stressed. An understanding of the antioxidant defense mechanisms in this kind of parasites, as well as their interactions with the specific host is crucial in the design of drugs targeting these organisms.

Selenocompounds in Cancer Therapy: An Overview

In vitro and in vivo experimental models clearly demonstrate the efficacy of Se compounds as anticancer agents, contingent upon chemical structures and concentrations of test molecules, as well as on the experimental model under investigation that together influence cellular availability of compounds, their molecular dynamics and mechanism of action. The latter includes direct and indirect redox effects on cellular targets by the activation and altered compartmentalization of molecular oxygen, and the interaction with protein thiols and Se proteins. As such, Se compounds interfere with the redox homeostasis and signaling of cancer cells to produce anticancer effects that include alterations in key regulatory elements of energy metabolism and cell cycle checkpoints that ultimately influence differentiation, proliferation, senescence, and death pathways. Cys-containing proteins and Se proteins involved in the response to Se compounds as sensors and transducers of anticancer signals, i.e., the pharmacoproteome of Se compounds, are described and include critical elements in the different phases of cancer onset and progression from initiation and escape of immune surveillance to tumor growth, angiogenesis, and metastasis. The efficacy and mode of action on these compounds vary depending on the inorganic and organic form of Se used as either supplement or pharmacological agent. In this regard, differences in experimental/clinical protocols provide options for either chemoprevention or therapy in different human cancers.

Systemic depletion of L-cyst(e)ine with cyst(e)inase increases reactive oxygen species and suppresses tumor growth

Cancer cells experience higher oxidative stress from reactive oxygen species (ROS) than do non-malignant cells because of genetic alterations and abnormal growth; as a result, maintenance of the antioxidant glutathione (GSH) is essential for their survival and proliferation. Under conditions of elevated ROS, endogenous L-cysteine (L-Cys) production is insufficient for GSH synthesis. This necessitates uptake of L-Cys that is predominantly in its disulfide form, L-cystine (CSSC), via the xCT(-) transporter. We show that administration of an engineered and pharmacologically optimized human cyst(e)inase enzyme mediates sustained depletion of the extracellular L-Cys and CSSC pool in mice and non-human primates. Treatment with this enzyme selectively causes cell cycle arrest and death in cancer cells due to depletion of intracellular GSH and ensuing elevated ROS; yet this treatment results in no apparent toxicities in mice even after months of continuous treatment. Cyst(e)inase suppressed the growth of prostate carcinoma allografts, reduced tumor growth in both prostate and breast cancer xenografts and doubled the median survival time of TCL1-Tg:p53-/- mice, which develop disease resembling human chronic lymphocytic leukemia. It was observed that enzyme-mediated depletion of the serum L-Cys and CSSC pool suppresses the growth of multiple tumors, yet is very well tolerated for prolonged periods, suggesting that cyst(e)inase represents a safe and effective therapeutic modality for inactivating antioxidant cellular responses in a wide range of malignancies.

Redox dysregulation, neuroinflammation, and NMDA receptor hypofunction: A "central hub" in schizophrenia pathophysiology?

Accumulating evidence points to altered GABAergic parvalbumin-expressing interneurons and impaired myelin/axonal integrity in schizophrenia. Both findings could be due to abnormal neurodevelopmental trajectories, affecting local neuronal networks and long-range synchrony and leading to cognitive deficits. In this review, we present data from animal models demonstrating that redox dysregulation, neuroinflammation and/or NMDAR hypofunction (as observed in patients) impairs the normal development of both parvalbumin interneurons and oligodendrocytes. These observations suggest that a dysregulation of the redox, neuroimmune, and glutamatergic systems due to genetic and early-life environmental risk factors could contribute to the anomalies of parvalbumin interneurons and white matter in schizophrenia, ultimately impacting cognition, social competence, and affective behavior via abnormal function of micro- and macrocircuits. Moreover, we propose that the redox, neuroimmune, and glutamatergic systems form a "central hub" where an imbalance within any of these "hub" systems leads to similar anomalies of parvalbumin interneurons and oligodendrocytes due to the tight and reciprocal interactions that exist among these systems. A combination of vulnerabilities for a dysregulation within more than one of these systems may be particularly deleterious. For these reasons, molecules, such as N-acetylcysteine, that possess antioxidant and anti-inflammatory properties and can also regulate glutamatergic transmission are promising tools for prevention in ultra-high risk patients or for early intervention therapy during the first stages of the disease.

Thioredoxin reductase 1 suppresses adipocyte differentiation and insulin responsiveness

Recently thioredoxin reductase 1 (TrxR1), encoded by Txnrd1, was suggested to modulate glucose and lipid metabolism in mice. Here we discovered that TrxR1 suppresses insulin responsiveness, anabolic metabolism and adipocyte differentiation. Immortalized mouse embryonic fibroblasts (MEFs) lacking Txnrd1 (Txnrd1^−/−) displayed increased metabolic flux, glycogen storage, lipogenesis and adipogenesis. This phenotype coincided with upregulated PPARγ expression, promotion of mitotic clonal expansion and downregulation of p27 and p53. Enhanced Akt activation also contributed to augmented adipogenesis and insulin sensitivity. Knockdown of TXNRD1 transcripts accelerated adipocyte differentiation also in human primary preadipocytes. Furthermore, TXNRD1 transcript levels in subcutaneous adipose tissue from 56 women were inversely associated with insulin sensitivity in vivo and lipogenesis in their isolated adipocytes. These results suggest that TrxR1 suppresses anabolic metabolism and adipogenesis by inhibition of intracellular signaling pathways downstream of insulin stimulation.

Sulfasalazine intensifies temozolomide cytotoxicity in human glioblastoma cells

Temozolomide (TMZ) is an alkylating agent used to treat glioblastoma. This tumor type synthesizes the antioxidant glutathione through system X c (-) , which is inhibited by sulfasalazine (SAS). We exposed A172 and T98G human glioblastoma cells to a presumably clinically relevant concentration of TMZ (25 µM) and/or 0.5 mM SAS for 1, 3, or 5 days and assessed cell viability. For both cell lines, TMZ alone did not alter viability at any time point, while the coadministration of TMZ and SAS significantly reduced cell viability after 5 days. The drug combination exerted a synergistic effect on A172 cells after 3 and 5 days. Therefore, this particular lineage was subjected to complementary analyses on the genetic (transcriptome) and functional (glutathione and proliferating cell nuclear antigen (PCNA) protein) levels. Cellular pathways containing differentially expressed genes related to the cell cycle were modified by TMZ alone. On the other hand, SAS regulated pathways associated with glutathione metabolism and synthesis, irrespective of TMZ. Moreover, SAS, but not TMZ, depleted the total glutathione level. Compared with the vehicle-treated cells, the level of PCNA protein was lower in cells treated with TMZ alone or in combination with SAS. In conclusion, our data showed that the association of TMZ and SAS is cytotoxic to T98G and A172 cells, thus providing useful insights for improving TMZ clinical efficacy through testing this novel drug combination. Moreover, the present study not only reports original information on differential gene expression in glioblastoma cells exposed to TMZ and/or SAS but also describes an antiproliferative effect of TMZ, which has not yet been observed in A172 cells.

Mechanisms of ferroptosis

Ferroptosis is a non-apoptotic form of cell death that can be triggered by small molecules or conditions that inhibit glutathione biosynthesis or the glutathione-dependent antioxidant enzyme glutathione peroxidase 4 (GPX4). This lethal process is defined by the iron-dependent accumulation of lipid reactive oxygen species and depletion of plasma membrane polyunsaturated fatty acids. Cancer cells with high level RAS-RAF-MEK pathway activity or p53 expression may be sensitized to this process. Conversely, a number of small molecule inhibitors of ferroptosis have been identified, including ferrostatin-1 and liproxstatin-1, which can block pathological cell death events in brain, kidney and other tissues. Recent work has identified a number of genes required for ferroptosis, including those involved in lipid and amino acid metabolism. Outstanding questions include the relationship between ferroptosis and other forms of cell death, and whether activation or inhibition of ferroptosis can be exploited to achieve desirable therapeutic ends.

Regulated necrosis: disease relevance and therapeutic opportunities

The discovery of regulated cell death presents tantalizing possibilities for gaining control over the life-death decisions made by cells in disease. Although apoptosis has been the focus of drug discovery for many years, recent research has identified regulatory mechanisms and signalling pathways for previously unrecognized, regulated necrotic cell death routines. Distinct critical nodes have been characterized for some of these alternative cell death routines, whereas other cell death routines are just beginning to be unravelled. In this Review, we describe forms of regulated necrotic cell death, including necroptosis, the emerging cell death modality of ferroptosis (and the related oxytosis) and the less well comprehended parthanatos and cyclophilin D-mediated necrosis. We focus on small molecules, proteins and pathways that can induce and inhibit these non-apoptotic forms of cell death, and discuss strategies for translating this understanding into new therapeutics for certain disease contexts.

Thiol-Disulfide Exchange Reactions in the Mammalian Extracellular Environment

Disulfide bonds represent versatile posttranslational modifications whose roles encompass the structure, catalysis, and regulation of protein function. Due to the oxidizing nature of the extracellular environment, disulfide bonds found in secreted proteins were once believed to be inert. This notion has been challenged by the discovery of redox-sensitive disulfides that, once cleaved, can lead to changes in protein activity. These functional disulfides are twisted into unique configurations, leading to high strain and potential energy. In some cases, cleavage of these disulfides can lead to a gain of function in protein activity. Thus, these motifs can be referred to as switches. We describe the couples that control redox in the extracellular environment, examine several examples of proteins with switchable disulfides, and discuss the potential applications of disulfides in molecular biology.